A medium having properties which do not exist in nature can be artificially structured by arraying small pieces of a metal, a dielectric substance, a magnetic material and a superconductive material at sufficiently shorter spacings than a wavelength (about one-tenth or less of wavelength). This medium is referred to as a metamaterial in the sense of being beyond media in nature. The properties of the metamaterial vary depending on shape, material property and arrangement of unit particles. In particular, a metamaterial having an equivalent dielectric constant ∈ and a magnetic permeability μ simultaneously showing a negative value has been called a “left-handed medium (Left-Handed Materials)”, because its electric field, magnetic field and wave vector form a left-handed system. In contrast to this, a normal medium having an equivalent dielectric constant ∈ and a magnetic permeability μ simultaneously showing a positive value is referred to as a “right-handed medium (Right-Handed Materials)”. In particular, the “left-handed medium” was predicted by a Russian physicist, Veselago in 1967 to have peculiar properties such as so-called a backward-wave which is a wave having a group velocity (propagation velocity of energy) having an opposite sign to a phase velocity (progression velocity of phase) and an amplification of an evanescent wave which exponentially decays in a nonpropagation region. A relational region of the dielectric constant ∈, the magnetic permeability μ and the medium can be classified into the media of the first to fourth quadrants, according to positive and negative values of the dielectric constant ∈ and positive and negative values of the magnetic permeability μ, as shown in FIG. 1.
Some left-handed media have been conventionally invented, and representative media will now be exemplified below. For instance, a document according to D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, “Composite medium with simultaneously negative permeability and permittivity” Phys. Rev. Lett., vol. 84, no. 18, pp. 4184-4187, May 2000 (Document 1) describes a left-handed artificial medium which is composed of unit cells each consisting of a split-ring resonator and a wire resonator, and shows that the medium shows left-handed characteristics experimentally. The medium is referred to as a resonant type left-handed medium, because the structure uses the resonator. The medium shows an extremely large loss due to the resonance and an extremely narrow operational band, because the medium can work only in the vicinity of the resonance frequency of the split-ring resonator and the wire resonator.
Among one-dimensional nonresonant type left-handed media achieving left-handed characteristics without using a resonator, a medium using a microstrip line is described in a document according to C. Caloz and T. Itoh, “Application of the transmission line theory of left-handed (LH) materials to the realization of a microstrip LH Line”, IEEE-APS Int'l Symp. Digest, vol. 2, pp. 412-415, June 2002 (Document 2), or a medium having a one-dimensional structure based on a coplanar waveguide (Coplanar Waveguide: CPW) is described in a document according to A. Grbic and G. V. Eleftheriades, “Experimental verification of backward-wave radiation from a negative refractive Index metamaterial”, Journal of Applied Physics, Vol. 92, No. 10, pp. 5930-5935, November 2002 (Document 3).
The medium based on the microstrip line achieves the left-handed characteristics from series capacitance between adjacent metallic patterns of unit cells and parallel inductance generated from vias which connect the metallic patterns with a ground plane. However, the medium has had a difficulty in reducing the size of unit cells and integrating the unit cells because the density of the vias which penetrate a substrate is restricted from a production viewpoint. The medium also has had a problem of requiring a high manufacturing cost, because of needing not only to process a substrate surface solely, but also to prepare a metal which penetrates the substrate. On the other hand, the one-dimensional left-handed medium using the coplanar waveguide can obtain the parallel inductance even without using the via which penetrates the substrate because a ground conductor is arranged in the same plane as a signal metal, but it has been difficult to form a two-dimensional medium.
In addition, as for two-dimensional nonresonant type left-handed media, a medium composed of series capacitance and parallel inductance produced by using an LC lumped-parameter element is described in a document according to V. Eleftheriades, A. K. Iyer and P. C. Kremer, “Planar Negative Refractive Index Media Using Periodically L-C Loaded Transmission Lines”, IEEE Transactions on Microwave Theory and Techniques, Vol. 50, No. 12, pp. 2702-2712, December 2002 (Document 4), and a distributed-constant-type medium composed of only a metallic pattern is described in a document according to Atsushi Sanada, Christophe Caloz and Tatsuo Itoh, “Planar Distributed Structures with Negative Refractive Index”, IEEE Trans. on Microwave Theory and Techniques, Vol. 52, No. 4, pp. 1252-1263, April 2004 (Document 5). Any of the media uses an inductor chip or a via (through hole) which penetrate a substrate for forming the parallel inductance with respect to a ground plane, and accordingly has the above described problems.
A medium has been known which uses a one-dimensional line devised so as to possess a left-handed characteristic without using a via. The medium has a structure of connecting parallel inductance with a large-area metallic patch having a large ground capacitance, instead of directly connecting with a ground plane of a rear surface of a substrate through the via, and has been well known as described in a document, for instance, according to Atsushi Sanada, Koichi Murakami, Shuji Aso, Hiroshi Kubo, and Ikuo Awai, “A via-free microstrip left-handed transmission line”, IEEE International Microwave Symposium Digest, pp. 301-304, Fort Worth, June 2004 (Document 6). However, the structure is not suitable for integration because of requiring a large-area metallic patch. In addition, the structure has not been extended to a two-dimensional medium.
As described above, a conventional medium needs to prepare a via (through hole) which penetrates a substrate and connects the upper surface of a substrate with a ground plane to form an inductance, and accordingly has had a difficulty in integrating unit cells because an upper limit of the density of the via is limited. The conventional medium also has had a problem of requiring a high manufacturing cost, because of needing not only to process a substrate surface solely, but also to prepare a metal which penetrates the substrate.